The specific activities of aldolase of cytosols from the livers of 30-31 month old rats were statistically significantly lower than those isolated from 6 or 12 month old animals. Although these data provide consistent evidence for the presence of inactive aldolase in the cytosols isolated from old rats in both studies of immunoprecipitation and purification of the enzyme, the age-associated changes may be considered of marginal importance. For example, in the former studies between 13 and 22% inactive enzymatic molecules were observed. In the latter studies estimates of their presence were similar; namely; 15%. A greater increase in aldolase activity occurs in mice as compared to rats fed either a high fructose-no fat diet or a low protein-high carbohydrate diet. This latter finding suggests that there is a difference in the dietary protein requirements of rats and mice. Indeed, the growth curves established in this study clearly indicated that the mouse can support growth more readily on low levels of dietary protein than the rat.